bbPress.org

Forums, topics and replies live in the posts table (typically wp_posts but the prefix might be different in your case). Something like this may help:

UPDATE wp_posts
SET    post_author = 1
WHERE  post_author = 0
AND    post_type IN ( 'reply', 'topic' )

Essentially it finds all replies and topics where the author ID is zero and updates them to be associated with some other user (I used user ID 1 in my example, obviously modify to suit).